The present invention relates to an active matrix substrate for a liquid crystal display device and a method of manufacturing the same.
Liquid crystal display devices (LCD) are advantageous since they are formed thin and display color images with low power consumption. By virtue of these advantages, the LCDs are widely used for lap-top personal computers. The image quality of the LCDs is good enough to be employed not only for electric-data display devices but also for TV screens.
Of the LCD devices, an active matrix type LCD is used as a flat panel display capable of providing full-color images with a high quality. The active matrix type LCD is formed of a first glass substrate, a second glass substrate and liquid crystal which is injected between the first and the second glass substrate. In the first glass substrate, thin transistors (TFT), which employ amorphous silicon or poly crystalline silicon as an active layer, are arranged in a matrix form. The second glass substrate is fixed so as to face the first glass substrate with a gap of about 5 μm interposed between them.
FIG. 1 shows a cross-sectional view of a pixel portion of a conventional active matrix type LCD. A scanning line 3502 and a storage capacitor line 3503 are formed on a glass substrate 3501. A gate insulating film 3504 is formed over the lines 3502 and 3503. Furthermore, a pixel electrode 3505 is selectively formed on the gate insulating film 3504.
Reference numeral 3506 is a TFT portion, which is formed of a semiconductor layer 3507, a channel protecting insulating film 3508 formed on the semiconductor layer 3507, and two doped semiconductor layers 3509 facing each other. The two doped semiconductor layers are formed in contact with the semiconductor layer 3507 while an end portion of each of the doped semiconductor layers is mounted on the channel protecting insulating film 3508. A source electrode 3510 and a drain electrode 3511 are formed respectively on the two doped semiconductor layers 3509. The source electrode 3510 is connected to a signal line (not shown). The drain electrode 3511 is connected to the pixel electrode 3505. A protective insulating film 3512 is formed over the TFT portion 3506.
With recent technical development, a field of view has been widened. Accordingly, a narrow viewing angle of the LCD has been overcome. In addition to this, since the TFT array can be formed on the glass substrate, a relatively large display having a diagonal length of about 10 to 25 inches has been realized.
However, to realize a high definition TV (HDTV), a large screen having a diagonal length of about 40–60 inches is desired. To manufacture the TFT array for such a large screen, it is necessary to construct an assembly line capable of holding an ultra-large glass substrate larger than 1 m square. A large equipment cost is inevitably required.
A method of making the large screen by jointing a plurality of substrates carrying TFT arrays is disclosed in Japanese Patent Application KOKAI publication No. 10-268332. However, this method has the following problems. Since the substrates are not jointed accurately, an aperture ratio of the joint portion is low. It is difficult to accurately control the level of the joint portion between the substrates, taking the thickness (5 μm) of the liquid crystal layer into consideration. Therefore, a large quantity of the substrates are not manufactured.
On the other hand, a mobile data terminal equipment providing electronic data anytime and anywhere was developed by making use of “low power consumption” of the LCD. The mobile data terminal equipment has been used in a wide variety of fields. In future, it is expected that electronic data will be displayed with the same ultra precision as that of printing matter, that is, about 150–300 pixel/inch (ppi).
These mobile data terminal equipments have to be formed light with a low power consumption. When a liquid crystal display is formed on an A4-size glass substrate of about 0.7 mm-thick, the total weight of the display results in 220 g. If the weight of the bezel for fixing the display is included, the total weight of the device will be about 400 g or more.
The weight of the display device can be reduced by about ½ if a plastic substrate is employed. The weight can be further reduced, if a film substrate is used. Such a display device is suitable for use in the mobile data terminal equipment. In these circumstances, attempts have been made to form the TFTs on the plastic substrate or the film substrate. When the TFTs are formed on these substrates, however; it is necessary to reduce the processing temperature. If the TFTs are formed at a low processing temperature, performance of the TFTS may be degraded, with the result that limitations may be imposed on image quality and the number of pixels. Furthermore, the thermal expansion coefficiency of these substrates is high and plastic deformation occurs at a low temperature. For these reasons, it is conceivable that the high definition display device may not be attained.